The present invention relates to a travelling course searching apparatus for vehicle, which is capable of searching an optimal course connecting with each other two points preset on a road map by using the road map data.
The Japanese laid open patent publications No. 133600-87 and No. 173297-89 disclose such a vehicle travel guiding apparatus which is capable of setting any start and target points on a road map shown on a display screen and searching an optimal course therebetween on the basis of the digitized map data to guide the vehicle.
A basic method for searching an optimal course on the basis of the digital map data consists in that nodes of line segments connecting a start point to a target point on a road map are sequentially searched according to a known algorithm such as "Dijkstra" so as to obtain the shortest distance therebetween.
However, today's road maps contain an increased quantity of road data other than road distances, which may be applied for finding further optimal course from a start point to a target point in view of road parameters other than the shortest distance. Therefore, a prior art disclosed in the Japanese laid open patent publications No. 138409-89 is to search a vehicle's travelling course according to a function for evaluating travel costs relating to such variable parameters as a total distance from a start point to a target point and a mean travel time to be required, a road width, a number of turns of roadway and so on, and thereby to find an optimal course of the least cost. A synthetically evaluating function Ci for a portion i of a travel course is, by way of example, expressed as follows: EQU Ci=k1.multidot.li+k2.multidot.ti+k3.multidot.wi+k4.multidot.ni.sup.2( 1)
where li--a distance cost, ti--a mean travel time cost, wi--a road width cost, k1 to k4--weight coefficients of respective costs on the condition that k1, k2, k3, k4.gtoreq.0.
Since the wider road may decrease a travelling cost, the road width cost wi at a distance li and a mean road width Wi can be expressed as wi=li/Wi.
The equation (1) means that the travelling cost of the course portion may increase as the distance and the mean travelling time increase and may decrease as the road width increases (making easier to drive the vehicle). If the number of turns of the course portion is small, it may scarcely affect the travelling cost. But, a large number of turns causes a heavy load to the driver, thereby sharply increasing the travelling cost.
In reverse to the above-mentioned evaluating function it is also possible to induce such an evaluating function that an optimal course may be of the maximum value.
The introduction of such a synthetically evaluating function makes it possible to judge which one among different courses shall be selected in view of a number of road parameters such as distance, mean travelling time, road width, the number of turns of a traveling course and the like by comparing the values according to the function.
However, in case of searching an optimal course from a start point to a target point by comparing costs of parameters by means of the evaluating function such a problem may occur that preset constant weight coefficients of variable parameters of the function are not always adequate to local searching conditions.
As shown in FIG. 3, for example, when a driver wishes to travel from a start point S to a target point O on a road map by selecting a main road (highway) which has generally excellent travelling cost parameters as to distance and road width, travel course searching shall be done in such a way that a highway MR extending near from the start point S and near to the target point O is first selected and then a local path PR1 connecting the start point S to the highway MR and a local path PR2 connecting the highway MR to the target point O is searched.
If in this case a travelling course of least cost value is searched according to an equation (2) of the synthetically evaluating function C1 which, for example, has a weight coefficient 1.0 of distance cost l and a weight coefficient 0.1 of road width cost w, and the function C1 is applied to, for example, a course portion of distance l=100 m and road width W=5 m, the distance cost is determined as 1.0.times.100 m=100 and the road width cost is determined as 0.1.times.(100 m/5 m)=2. Application of such function regarding the distance higher than the road-width may result in selecting a narrow local road LR instead of the highway MR as shown in FIG. 4. EQU C1=1.0.times.l+0.1.times.(l/W) (2)
If a function C2 having a decreased distance cost l and an increased road-width cost W as expressed by an equation (3) is applied for searching a course portion of 100 m in distance and 5 m in road width, the distance cost is determined as 0.5.times.100 m=50 and road width cost is determined as 1.0.times.(100 m/5 m)=20. When a travelling course of the least cost is searched by use of the function C2 having the considerably weighted road-width cost, a highway MR may be selected but longer roadways PR1' and PR2' connecting the highway, respectively, to the start point S and the target point O may also be selected because of regarding the road width higher than the distance, as shown in FIG. 5. EQU C2=0.5.times.l+0.1.times.(l/W) (3)